1. Field of the Invention
This invention relates to aqueous dispersions of water insoluble copolymers stabilized by multiple carboxylic acid functional comonomer residues against coagulation resulting from mechanical shear. These compositions display improved mechanical stability in comparison with dispersions stabilized by monocarboxylic acid functional comonomer residues. These compositions are useful as pressure sensitive adhesives and as binders for textiles, paper, nonwoven materials, and coatings.
2. Brief Description of the Prior Art
Aqueous dispersions of water insoluble copolymers which are stabilized by chemically bound carboxyl groups are known to exhibit better mechanical stability, freeze-thaw stability, and pigment compatibility than dispersions stabilized only with physically adsorbed soaps. K. Sakota and T. Okaya, J. of Applied Poly. Science, 20 1735-1744 (1976). The stability of dispersions carboxylated by the inclusion of monocarboxylic acid functional comonomer has been investigated as a function of polymerization process. K. Sakota and T. Okaya, J. of Applied Poly. Science, 21 1025-1034 (1977); 20 2583-2587 (1976). It is known that surface bound acid is more effective in promoting stability than acid buried within the latex particles. D. R. Vijayendran, J. of Poly. Science, 23 893-11 (1979), has shown that the hydrophobicity of the carboxylic acid functional comonomer relative to that of the comonomers is important in determining to what extent the carboxylic acid functional comonomer will be buried or surface bound in the latex particle or will fail to incorporate into the latex particle altogether. It was found that a dicarboxylic acid functional comonomer, itaconic acid, largely failed to incorporate into a polystyrene latex in comparison with the more hydrophobic monocarboxylic acid functional comonomers, methacrylic and acrylic acids. In addition to the effect on mechanical stability, the nature of the carboxylic acid functional comonomer can affect a variety of other properties of the latex particle and films formed by coalescing latex particles. K. L. Hoy, J. of Coatings Technology, 51 27-41 (1981).
Monocarboxylic acid functional monomers such as acrylic and methacrylic acid are well known as constituents of pressure sensitive adhesive compositions. H. Warson, The Applications of Synthetic Resin Emulsions, 312 (1972). D. Satas, Handbook of Pressure Sensitive Adhesive Technology, 299-305 (1982). Similarily, the use of dicarboxylic acid functional comonomers such as maleic, fumaric, crotonic, itaconic, and citraconic acids and corresponding anhydrides such as maleic, itaconic and citraconic anhydrides, as constituents of pressure-sensitive adhesives, is known to the art.
In order to be useful as or in formulating pressure sensitive adhesives (PSAs), it is desirable that copolymers be tacky at the temperature of use. Assuming constant molecular weight and the absence of unreacted comonomer, solvent and plasticizer, tackiness can be achieved by selecting a copolymer composition displaying a low glass transition temperature (T.sub.g). A T.sub.g below 0.degree. C. is desirable for adhesives which are to be used at or near room temperature. In acrylic and other adhesives in which low copolymer T.sub.g is achievable be selecting comonomers with relatively long alkyl side chains (e.g., alkyl esters of acrylic and methacrylic acids), low T.sub.g copolymer is characterized by relatively great hydrophobicity. Prior art difunctional carboxylic acids are relatively hydrophilic and may consequently be difficult to incorporate in tacky, hydrophobic PSA copolymers.
The multifunctional acid comonomers of the present invention are useful in stabilizing hydrophobic PSA copolymers.
Latex rheological properties are sensitive to the incorporation of ionizable comonomers such as carboxylic functional monomers. For example, W. J. Lee, in U.S. Pat. No. 3,458,466, granted July 29, 1969, discloses latex compositions having improved rheological properties prepared from a comonomer composition comprising from about 1 to 10% of an alpha, beta-monoethylenically unsaturated mono- or polycarboxylic acid monomer having from 3 to 8 carbon atoms.
In one embodiment of this invention, a multifunctional acid anhydride is reacted with a hydroxyalkyl(meth)acrylate to yield comonomer useful in practicing the invention. The reaction of an alcohol functional compound with an anhydride is well known in the chemical art. For example, D. H. Clemens in U.S. Pat. No. 3,150,118, granted Sept. 22, 1964, discloses unsaturated acidic monomers produced by heating a hydroxyalkyl acrylate or methacrylate with maleic, succinic, or phthalic anhydride. Copolymers containing from about 1/4 to 10% by weight of the acidic monomers are disclosed to be particularly useful in the production of emulsion and solution coating materials.
Some of the preferred comonomers employed in the present invention are known in the chemical art. For example, I. H. McEwan in Canadian Patent No. 729,729, issued Mar. 8, 1966, discloses copolymers comprising copolymerized ethylenically unsaturated esters of the formula EQU X--C(O)O--CHY--C(Z).dbd.CH.sub.2
wherein X is an organic radical containing at least one free hydroxyl or carboxyl group and each of Y and Z is selected from the group consisting of hydrogen, halogen, hydroxyl and alkyl radicals of up to three carbon atoms. Examples of such comonomers are the allyl, methallyl, chloroallyl, hydroxyallyl and 1-penten-3-yl esters of acids such as succinic, glutaric, adipic, phthalic, mellitic, and trimellitic acids. Coating compositions containing a copolymer prepared with such acid functional monomer and a crosslinking agent and preferably a crosslinking catalyst are disclosed.
P. J. Pare in U.S. Pat. No. 3,207,765, granted Sept. 21, 1965, discloses a process for the production of vinyl trimellitates and polyester resins containing such trimellitates which may be crosslinked by admixing with an alpha, beta-unsaturated polymerizable monomer such as styrene, allylmethacrylate, or triallylphosphate and an optional peroxide catalyst.
J. R. Stephens and R. E. Van Strien in U.S. Pat. No. 3,063,969, granted Nov. 13, 1962, disclose 4-vinyltrimellitate and 4-vinyltrimellitate anhydride as well as polymers and copolymers prepared using these monomers which are useful for coatings and thermosetting resins.
H. Yasuno et al. in U.S. Pat. No. 4,250,007, granted Feb. 10, 1981, discloses a photosetting composition useful as a coating varnish comprising a modified, epoxidized butadiene polymer comprising 5-50 aromatic or cycloaliphatic polybasic carboxylic ester side chain groups per 100 butadiene units, each having at least one acryloyl or methacryloyl terminal group and a linking group formed by a reaction between the carboxylic radical in the polybasic carboxylic acid and an epoxy radical in the epoxidized butadiene polymer. Polybasic carboxylic acid compounds prepared from condensation from 2-hydroxyethyl acrylate and phthalic anhydride, 2,3,3',4-biphenyltetracarboxylic dianhydride, and trimellitic anhydride are also disclosed.
Y. Taneda et al. in U.S. Pat. No. 3,695,877, granted Oct. 3, 1972, disclose photopolymerizable resin compositions comprising unsaturated polyester obtained by polycondensation of an acid component comprising (1) at least 5 mole percent (with respect to the total acid component) of aromatic polycarboxylic acid containing at least 3 carboxylic groups or functional derivatives thereof and (2) at least 30 mole percent (with respect to the total acid component) of ethylenically unsaturated dicarboxylic acid or functional derivative thereof. A number of tricarboxylic acid functional aromatic compounds such as hemimellitic acid, trimellitic acid, 2,4,5-toluene tricarboxylic acid, pyromellitic acid and 1,2,4-naphthalene tricarboxylic acid are also disclosed.
E. Masuhara et al. in Reports of the Institute for Medical and Dental Engineering, 13, 21-25 (1979) disclose the preparation of 4-methacryloxyethyl trimellitic anhydride and its use as a comonomer with methylmethacrylate to improve adhesion between polymethyl methacrylate rod and hard tissue such as human enamel.